The goal of the doctoral dissertation was to investigate the factors and steps in the PWV assessment protocol - the gold standard for arterial stiffness measurements, which can affect the reduction of unwanted PWV variability within subjects and thus facilitate the introduction of PWV into clinical practice. Furthermore, the factors that can lead to technical difficulties in measuring PWV were investigated and identified, and the possibility of the impact of a recent infection of COVID-19 on the state of blood vessels several months after recovery and its disruptive effect in longitudinal studies was also investigated. Analyses were performed separately for each of the two validated devices - the SphygmoCor applanation tonometer and the Arteriogaph oscillometer - which use different techniques for non-invasive PWV measurement, except for the study with subjects who had recently recovered from COVID-19 in which measurements were made only with the SphygmoCor. The results are based on two unique institutions of study in the area. In a study in which the aim was to identify the experimental conditions (different measuring devices, time of day and number of visits) and other confounding factors (external meteorological conditions, physiological parameters - arterial blood pressure and heart rate, and characteristics of the subjects - age, gender, hypertension status, BMI) that affect the measured PWV values and/or cause technical difficulties with PWV measurement, which are not controlled in the standard measurement protocol, for the first time the short-term variability of PWV within subjects was systematically examined. As clinically significant changes in arterial stiffness in an individual are not expected within a short observation period of 2 weeks, the intra-individual variability of PWV observed in the study represents an error in longitudinal measurements of PWV that can be expected in the clinic. The design used — a single-blind cross-over study with two different devices, randomized with regard to two different measuring devices and two different times of day (morning/aftemoon) with 12 repeated measurements per subject and subjects uniformly and widely distributed by age, BMI -in, gender and diagnosis of hypertension (normotensive/hypertensive) provides a high level of evidence. Based on the results of this study, a number of recommendations were proposed that could reduce unwanted intra-individual variability when assessing PMA" and facilitate the use of PWV in clinical practice. The second study is the only study to date that compared changes in arterial stiffness and central hemodynamics before and after infection with COVID-19. The results show that the values of the parameters continue to deteriorate in the period between 2 and 3 months after the end of the infection, suggesting that it is possible that the COVID-19 infection leads to a perrnanent deterioration of arterial stiffness. This consequently means an increase in the risk for the development of CVD, even in predominantly young, healthy individuals with a mild form of the disease, such as those described in this study. The results of this doctoral thesis, which include a number of recommendations for reducing unwanted intra-individual variability of PWV in an individual, as well as reducing the incidence of technical difficulties during measurement and identifying new confounding factors such as COVID-19, can facilitate the introduction of PWV measurement into clinical practice and interpreting longitudinal studies.